Acceleration sensor

ABSTRACT

An acceleration sensor (1) includes a parallelepiped case (2). A circuit board (3) is secured in the case (2). A sensor chip (5), on which a diffusion strain gauge (5b) is formed, is mounted on the circuit board (3). A cylindrical acceleration selecting element (13) is provided on the case (2). The axis of the acceleration selecting element (13) extends in the x direction. A cylindrical pressure transmitting element (14) is also provided on the case (2). The pressure transmitting element (14) communicates a hollowed part on the top of the sensor chip (5) with the interior of the acceleration selecting element (13). The hollowed part on the top of the sensor chip (5), the pressure transmitting element (14) and the acceleration selecting element (13) are filled with silicon gel (10).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acceleration sensor that detects anacceleration of an object to which the sensor is attached based onpressure changes caused by the inertia of the medium filling the sensor.

2. Description of the Related Art

There have been proposed various types of acceleration sensors fordetecting the acceleration of an object to which the sensor is attachedby detecting pressure changes. An acceleration sensor 70 illustrated inFIG. 16 is an example of such a sensor. The acceleration sensor 70includes a cylindrical sensor mount 72 mounted on a circuit board 71. Adiaphragm type semiconductor pressure sensitive sensor chip (hereinaftersimply referred to as a sensor chip), on which diffusion gauges 73 areformed, is provided in the sensor mount 72. Silicon gel 75 having arequired density fills the interior of the sensor mount 72 to cover thesensor chip 74. In the acceleration sensor 70 having the abovestructure, acceleration of an object to which the sensor 70 is attachedcauses the silicon gel 75 to fluctuate in response to the acceleration.The pressure caused by the fluctuation of the silicon gel 75 istransmitted to the sensor chip 74 and therefore strains the sensor chip74. A force is applied to the diffusion strain gauge 73 in accordancewith the strain. The diffusion strain gauge 73 then senses the forceapplied thereto and outputs an electrical signal in accordance with theacceleration as a detection signal. In other words, since the sensorchip 74 is subjected to the pressure of the silicon gel 75, the sensorchip 74 is therefore most susceptible to the pressure caused byacceleration in the vertical direction with respect to the circuit board71 (direction z in the figures). The principle detectable direction ofthe acceleration sensor 70 is therefore its vertical direction(direction z in the figures).

FIG. 17 shows another acceleration sensor 80. In the acceleration sensor80, a weight portion 84 is attached to a silicon substrate 81 by acantilever 82. A diffusion strain gauge 83 is provided at the proximalportion of the cantilever 82. The silicon substrate 81 surrounds theweight portion 84 with a predetermined space therebetween. In theacceleration sensor 80 of the above structure, acceleration of an objectto which the sensor 80 is attached causes the weight portion 84 to bendin response to the acceleration. This strains the proximal portion ofthe cantilever 82. A force is applied to the diffusion strain gauge 83in accordance with the strain. The diffusion strain gauge 83 then sensesthe force and outputs an electrical signal in accordance with theacceleration as a detection signal. The weight portion 84 thereforebends most when the object and the acceleration sensor 80 areaccelerated in the vertical direction with respect to the weight portion84 (direction z in the figures). This allows a strong detection signalto be produced. The principle detectable direction of the accelerationsensor 80 is therefore its vertical direction (direction z in thefigures).

There has been also proposed a three dimensional acceleration sensor fordetecting accelerations in a vertical direction (z direction) as well ashorizontal directions (x and y directions). FIGS. 18 and 19 show such anacceleration sensor 90. The acceleration sensor 90 has a weight portion91 having a shape of a truncated pyramid supported by cantilevers 92 to95. The cantilevers 92 to 95 have diffusion strain gauges 96 to 99 ontheir top surfaces, respectively. In the acceleration sensor 90 of theabove structure, as in the case of the acceleration sensor 80,acceleration of an object to which the sensor 80 is attached causes theweight portion 91 to bend in response to the acceleration. This strainsthe cantilevers 92 to 95. Force is applied to the diffusion straingauges 96 to 99 in accordance with the strain. The diffusion straingauges 96 to 99 then sense the force applied thereto and issue detectionsignals in accordance with the acceleration of the object and theacceleration sensor 90, respectively.

Japanese Unexamined Patent Publication 1-245163 discloses anotheracceleration detection device. The acceleration detection device has anelastic plate and a weight portion fixed to the central portion of theplate. The weight portion extends vertically with respect to the plate.Acceleration of the acceleration detection device causes the weightportion to sway. This strains the elastic plate. A strain detectionmeans provided on the elastic plate detects the strain. An externalforce detector then detects the acceleration applied to the weightportion.

Japanese Unexamined Patent Publication 1-253656 discloses anotheracceleration sensor. This acceleration sensor has an inertia weightportion and a piezo-resistance effect type pressure sensitive part. Theweight portion converts accelerations into inertia pressure. Thepiezo-resistance effect type pressure element detects the stress of adiaphragm, which senses the inertia pressure. The inertia weight portionis constituted by mercury. Acceleration applied to this accelerationsensor is converted to an inertia pressure by the mercury. The pressuredetection element detects the inertia pressure for detecting theacceleration.

Incidentally, the above described acceleration sensors 70 and 80 aremainly designed for detecting acceleration in the z direction.Therefore, when detecting acceleration in an arbitrary direction, forexample, the x direction, the acceleration sensors 70 and 80 should bemounted in such a manner that the x direction is aligned with theprinciple detectable direction of the sensors. This limits the ways formounting the acceleration sensors 70 and 80 and therefore makes mountingthe sensors 70 and 80 troublesome.

Due to its complex structure, the manufacturing process of the weightportion 91 of the acceleration sensor 90 is complicated. The largeweight portion 91 may continue to fluctuate without any accelerationapplied thereto once it starts fluctuating. This results in badfrequency response characteristics. Furthermore, the detection signalsfrom the diffusion strain gauges 96 to 99 should be processed to betransmitted as detection signals corresponding to the x, y and zdirections, respectively. This requires a signal processor forprocessing the detection signals from the diffusion strain gauges 96 to99. This complicates the circuit structure.

The acceleration sensor disclosed in Japanese Unexamined PatentPublication 1-245163 requires a complicated process for forming theweight portion and attaching it to the elastic body. This increases themanufacturing cost. In the acceleration sensor disclosed in JapaneseUnexamined Patent Publication 1-253656, the diaphragms should bedirected in the x, y and z directions respectively. This complicates thesensor structure. The mercury must be sealed since it falls whendirected downward. Further, since mercury has no dampingcharacteristics, an abrupt acceleration may break the diaphragms.

It is possible to use the above described acceleration sensors 70 and 80for separately detecting acceleration in x, y and z directions. However,in this case, the acceleration sensors 70 and 80 need to be mounted inalignment with each of the x, y and z directions, respectively. Thisincreases the manufacturing cost, complicates the mounting process andenlarges the size of an entire sensor.

It is a primary objective of the present invention to provide anacceleration sensor that has a simple structure and detects accelerationin an arbitrary direction regardless of the mounting direction.

A further objective of the present invention is to provide anacceleration sensor that has a simple structure and detects anacceleration after dividing it into elements of multiple directions.

SUMMARY OF THE INVENTION

The acceleration sensor according to the present invention includes acase, a circuit board fixed to the case, and a pressure sensitive partfixed to the circuit board provided in the case. The case is filled witha medium that is chiefly constituted by a gel composition. Anacceleration selecting element for selecting an acceleration componentparallel to the board is provided on the case. The pressure sensitivepart detects a pressure generated by an acceleration component parallelto the circuit board, which element is selected by the accelerationselecting element. Accordingly, the present invention enables detectingthe pressure generated by an acceleration component parallel to theboard, which element is selected from an acceleration of the object bythe acceleration selecting element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a acceleration sensor according to afirst embodiment of the present invention;

FIG. 2 is a plan view of an acceleration sensor according to a secondembodiment of the present invention;

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2;

FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 2;

FIG. 5 is a cross-sectional view of an acceleration sensor according tothe third embodiment of the present invention;

FIG. 6 is a graph showing the characteristics of a conventionalacceleration sensor;

FIG. 7 is a graph showing the characteristics of an acceleration sensoraccording to a third embodiment of the present invention;

FIG. 8 is a plan view of an acceleration sensor according to a fourthembodiment of the present invention;

FIG. 9 is a cross-sectional view taken along line 9--9 of FIG. 8;

FIG. 10 is a cross-sectional view taken along line 10--10 of FIG. 8;

FIG. 11 is a plan view of an acceleration sensor according to a fifthembodiment of the present invention;

FIG. 12 is a cross-sectional view taken along line 12--12 of FIG. 11;

FIG. 13 is a plan view of an acceleration sensor according to a sixthembodiment of the present invention;

FIG. 14 is a cross-sectional view taken along line 14--14 of FIG. 13;

FIG. 15 is a cross-sectional view taken along line 15--15 of FIG. 13;

FIG. 16 is a cross-sectional view of a conventional acceleration sensor;

FIG. 17 is a cross-sectional view of a conventional acceleration sensor;

FIG. 18 is a perspective view of a conventional three dimensionalacceleration sensor;

FIG. 19 is a cross sectional view taken along line 19--19 of FIG. 18.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

An acceleration sensor according to a first embodiment of the presentinvention will now be described with reference to FIG. 1.

FIG. 1 is a cross-sectional view of an acceleration sensor 1 accordingto this embodiment.

The acceleration sensor 1 has a rectangular parallelepiped case 2. Acircuit board 3 is fixed in the case 2. The circuit board 3 has a bore4. A sensor chip 5 as a pressure sensitive part is attached to thebottom surface of the circuit board 3 so as to cover the bore 4.Pressure sensitive part refers to an element that detects pressureapplied thereto and converts the pressure into an electrical signal.

The sensor chip 5 is formed on a substrate made of semiconductormaterial, e.g., a silicon substrate. The top portion of the sensor 5forms a rectangular parallelepiped and is hollowed out to have atruncated pyramid-shaped recess. The hollowed part forms a thin wall 5a.The sensor chip 5 is generally called a diaphragm type semiconductorpressure sensitive sensor chip of a C-shaped cross section. A diffusionstrain gauge 5b is formed on the bottom surface of the thin wall 5a. Thediffusion strain gauge 5b detects strain at the thin wall 5a (in otherwords, pressure applied to the thin wall 5a) and outputs an electricalsignal in accordance with the pressure as a detection signal.

The sensor chip 5 is electrically coupled via bonding wires 6 to aprinted pattern (not shown) space formed on the circuit board 3. Asignal processor 7 incorporated in a common packaging (such as a surfacemounting type) is mounted on the circuit board 3. The signal processor 7is electrically connected via the printed pattern to the sensor chip 5.The signal processor 7 receives a detection signal from the sensor chip5, amplifies and processes the signal and transmits the signal. Externalterminals 8 are inserted in the case 2. The external terminals 8 arecoupled via bonding wires 9 to the printed pattern on the circuitpattern 3. The detection signal sent from the sensor chip 5 is amplifiedand processed by the signal processor 7 and sent to the outside of thesensor via the external terminals 8.

The interior of the case 2 is filled with silicon gel 10 consisting of agel composition. The silicon gel 10 covers the bottom surface of thecircuit board 3. In other words, the silicon gel 10 seals the sensorchip 5, the signal processor 7, the printed pattern, the bonding wires 6and 9 and the external terminals 8.

A lid 11 is fitted to the bottom of the case 2. An air hole 12 is formedin the lid 11.

An acceleration selecting element 13 and a pressure transmitting element14 are provided on the top surface of the case 2. The accelerationselecting element 13 has a cylindrical shape and placed in such a mannerthat its center line is directed in a predetermined direction (parallelto the circuit board 3 and the x direction in the figure). The pressuretransmitting element 14 has a cylindrical shape with its center linenormal to the circuit board 3 (the z direction in the figure). Thepressure transmitting element 14 is therefore formed normal to the thinwall 5a of the sensor chip 5. One end of the acceleration selectingelement (the left end in the figure) is connected to the pressuretransmitting element 14, while the other end (the right end in thefigure) opens in the x direction. The center line of the pressuretransmitting element 14 is coaxial with the center line of the bore 4formed in the circuit board 3. The interior of the accelerationselecting element 13 is communicated with the hollowed out part of thesensor chip 5 through the pressure transmitting element 14.

The hollowed out part in the top portion of the sensor chip 5, thepressure transmitting element 14 and the acceleration selecting element13 are filled with the silicon gel 10. The silicon gel 10 in theacceleration selecting element 13 is exposed in the x direction. Aweight 15 is located in the gel 10 in the vicinity of the exposedsurface.

The silicon gel 10 and weight 15 in the acceleration selecting element13 serve as an inertia body to an acceleration applied from the outside.The center line of the acceleration selecting element 13 is directed inthe x direction. In other words, when an object to which theacceleration sensor 1 is attached accelerates, the silicon gel 10 in theacceleration selecting element 13 is moved most by the x directioncomponent of the acceleration. The silicon gel 10 and the weight 15 inthe acceleration selecting element 13 fluctuate in accordance with the xdirection component of the object's acceleration. A pressure is appliedto the silicon gel 10 in the pressure transmitting element 14 inaccordance with the fluctuation. In other words, the accelerationselecting element 13 selects the x direction component in theacceleration of the object and the acceleration sensor 1 and convertsthe acceleration component into a pressure. The pressure converted bythe acceleration selecting element 13 is transmitted to the sensor chip5 by the silicon gel 10 filling the pressure transmitting element 14.When pressure is applied to the sensor chip 5, the thin wall 5a of thesensor chip 5 bends and vibrates in response to the pressure. Thediffusion strain gauge 5b detects the bending and the vibration of thethin wall 5a and issues an electrical signal corresponding to thebending and the vibration as a detection signal.

The mass of the silicon gel 10 filling the acceleration selectingelement 13, the mass of the weight 15 and the inner diameter of theacceleration selecting element 13 are represented by M₁₃, M₁₅ and R₁,respectively. The detection sensitivity Sx for detecting theacceleration component in x direction is proportionate to (M₃+M₁₅)/{2π(R₁ /2)² }.

Incident ally, the pressure transmitting element 14 is filled with thesilicon gel 10 like the acceleration selecting element 13. The centerline of the pressure transmitting element 14 extends in the z direction.Therefore, the silicon gel 10 in the pressure transmitting element 14 isfluctuated by an acceleration component in the z direction. The pressurecaused by the fluctuation of the gel 10 is applied to the thin wall 5aof the sensor chip 5. In other words, the thin wall sa of the sensorchip 5 receives the pressure of the fluctuation of the silicon gel 10filling the acceleration selecting element 13 and the weight 15 causedby an acceleration component in the x direction. The thin wall 5a of thesensor chip 5 also receives the pressure of the fluctuation of thesilicon gel 10 filling the pressure transmitting element 14 caused by anacceleration component in the z direction . The mass of the silicon gel10 filling the pressure transmitting element 14 is represented by M₁₄and the inner diameter of the pressure transmitting element 14 isrepresented by R₂. The detection sensitivity in the z direction Sz isproportionate to M₁₄ /{2π(R₂ /2)² }. Therefore, the relationship of thedetection sensitivity in the z direction to the detection sensitivity inthe x direction Sx is represented by the following formula;

    Sz/Sx≈ M.sub.14 /{2π(R.sub.2 /2).sup.2 }!/ M.sub.13 +M.sub.15)/{2π(R.sub.1 /2).sup.2 }!

Setting the detection sensitivity in the x direction Sx high relative tothe detection sensitivity in the z direction Sz enables selecting anddetecting only an acceleration component in the x direction. Therefore,the configuration of the acceleration selecting element 13 and thepressure transmitting element 14 satisfies the following formula;

    M.sub.14 /{2π(R.sub.2 /2).sup.2 }<<(M.sub.13 +M.sub.15)/{2π(R.sub.1 /2).sup.2 }

The right end of the acceleration selecting element 13 is covered with acap 16 so that dust and debris do not mix with the gel 10. The cap 16has an air hole 17. The exposed surface of the silicon gel 10 in theacceleration selecting element 13 is exposed to the outer air throughthe air hole 17.

When an object to which the above acceleration sensor 1 is attachedaccelerates, the silicon gel 10 filling the acceleration selectingelement 13 and the weight 15 are fluctuated by the accelerationcomponent in the x direction of the object and the accelerationsensor 1. The acceleration in the x direction is converted into pressurecorresponding to the acceleration in the x direction by the silicon gel10 and the weight 15 in the acceleration selecting element 13. Thepressure is transmitted to the sensor chip 5 via the silicon gel 10 inthe pressure transmitting element.

The thin wall 5a of the sensor chip 5, when receiving pressure, bendsand vibrates in accordance with the received pressure. The diffusionstrain gauge 5b formed on the wall 5a detects the bending and vibrationof the thin wall 5a and issues an electrical signal in accordance withthe strain caused by the bending and vibration (i.e., pressure appliedto the thin wall 5a) as a detection signal. The detection signal fromthe diffusion strain gauge 5b reflects only the acceleration componentof the direction selected by the acceleration selecting element, namelythe x direction, among the acceleration of the object and theacceleration sensor 1. The detection signal from the sensor chip 5 isamplified and processed by the signal processor 7 and transmitted to theoutside of the sensor.

As described above, in this embodiment, the acceleration selectingelement 13 is provided in the case 2. The acceleration selecting element13 selects the acceleration component in the x direction of anacceleration of the object to which the acceleration sensor 1 isattached. The pressure corresponding to the selected accelerationcomponent is transmitted to the sensor chip 5 via the accelerationtransmitting element 14 formed normal to the thin wall 5a of the sensorchip 5. The detection signal transmitted to the pressure is outputtedfrom the sensor chip 5.

As a result, a simple structure enables detecting an acceleration in anarbitrary direction regardless of the mounting direction of the sensorchip 5 since the direction of the sensor chip 5 does not have to beoriented in the same direction as the acceleration to be detected.

Furthermore, setting the centerline of the acceleration selectingelement 13 in a certain direction enables detecting the accelerationcomponent in the set direction.

(Second Embodiment)

A three dimensional acceleration sensor according to a second embodimentof the present invention will now be described with reference to FIGS. 2to 4.

To avoid a redundant description, like or same reference numerals in thefirst embodiment are given to corresponding parts of this embodiment.

FIG. 2 is a plan view of a three dimensional acceleration sensoraccording to a second embodiment of the present invention. FIG. 3 is across-sectional view taken along line 3--3 of FIG. 2. FIG. 4 is across-sectional view taken along line 4--4 of FIG. 2.

A three dimensional acceleration sensor 20 includes a circuit board 3having bores 4x to 4z. Sensor chips 5x to 5z are mounted on the bottomsurface of the circuit board 3 so as to cover the bores 4x to 4z,respectively.

A case 21 of the three dimensional acceleration sensor 20 hasacceleration selecting elements 13x, 13y and 13z, which are formedcylindrically like the acceleration selecting element in the firstembodiment. The center line of the acceleration selecting element 13x isparallel to the circuit board 3 (i.e., the x direction in FIGS. 2 and4). The center line of the acceleration selecting element 13y isparallel to the circuit board 3 (i.e., the y direction in FIGS. 2 and3). The centerline of the acceleration selecting element 13z is normalto the circuit board 3 (i.e., the z direction in FIGS. 3 and 4).

The case 21 also has pressure transmitting elements 14x to 14z. Thepressure transmitting elements 14x to 14z are each formed cylindricaland normal to the circuit board 3. The pressure transmitting elements14x to 14z are each formed normal to the sensor chips 5x to 5z,respectively.

The pressure transmitting elements 14x to 14z are connected to theacceleration selecting elements 13x to 13z, respectively. The interiorsof the acceleration selecting elements 13x to 13z is communicated withthe hollowed parts of the sensor chips 5x to 5z via the pressuretransmitting elements 14x to 14z, respectively. The hollowed parts ofthe sensor chips 5x to 5z, the acceleration selecting elements 13x to13z and the pressure transmitting elements 14x to 14z are each filledwith the silicon gel 10. Weights 15x to 15z are located in the silicongel 10 filling each of the acceleration selecting elements 13x to 13z asin the first embodiment.

The x direction acceleration component in an acceleration of an objectto which the three dimensional acceleration sensor 20 is attached isselected by the acceleration selecting element 13x and converted into apressure corresponding to the x-direction acceleration component. Theconverted pressure is transmitted to the sensor chip 5x by the silicongel 10 filling the pressure transmitting element 14x. The sensor chip 5xthen transmits a detection signal corresponding to the transmittedpressure, that is, the x direction acceleration component.

Likewise, the y direction acceleration component is selected by theacceleration selecting element 13y and converted into a pressurecorresponding to the y direction acceleration component. The convertedpressure is transmitted to the sensor chip 5y by the silicon gel 10filling the pressure transmitting element 14y. The sensor chip 5y issuesa detection signal corresponding to the transmitted pressure, that is,the y direction acceleration component.

The z-direction acceleration component is selected by the accelerationselecting element 13z and converted into a pressure corresponding to thez direction acceleration component. The converted pressure istransmitted to the sensor chip 5z by the silicon gel 10 filling thepressure transmitting element 14z. The sensor chip 5z transmits adetection signal corresponding to the transmitted pressure, that is, thez direction acceleration component.

The acceleration selecting elements 13x to 13z are each covered withcaps 16x to 16z so that dust and debris do not mix with the gel 10. Airholes 17x to 17z are formed in the caps 16x to 16z, respectively.

When an object to which the above three dimensional acceleration sensor20 is attached accelerates, the acceleration selecting element 13xselects the x direction acceleration component of the acceleration andconverts it into a pressure. The acceleration selecting element 13yselects the y direction acceleration component in the acceleration andconverts it into a pressure. The acceleration selecting element 13zselects the z direction acceleration component in the acceleration andconverts it into a pressure.

The converted pressures corresponding to the selected accelerationcomponents are transmitted to the sensor chips 5x to 5z via the pressuretransmitting elements 14x to 14z, respectively. The sensor chips 5x to5z detect the pressures and issue detection signals corresponding to thepressures, respectively. The signal processor 7 amplifies and processesthe detection signals and sends them to the outside of the sensor. Theacceleration components of the x, y and z directions are detectedseparately and a detection signal corresponding to each accelerationcomponent is produced.

As described above, in this embodiment, the acceleration selectingelements 13x to 13z are provided for separately selecting accelerationcomponents of the x, y and z directions. The sensor chips 5x to 5z areprovided for detecting pressures converted from the selectedacceleration components and producing the detection signals.

As a result, the detection signals, each corresponding to one of theacceleration components of the x, y and z directions, are produced. Thisenables separating and detecting a multi-dimensional acceleration by asimple structure.

The sensor chips 5x to 5z do not have to be oriented in x, y and zdirections, respectively, to detect the corresponding accelerationcomponents. The mounting of the sensor is thus facilitated. The sensorchips 5x to 5z are mounted on the same circuit board 3. This simplifiesthe structure of the sensor and reduces the manufacturing cost.

(Third Embodiment)

A third embodiment of the present invention will now be described withreference to FIG. 5.

To avoid a redundant description, like or same reference numerals in thefirst embodiment are given to corresponding parts of this embodiment.

FIG. 5 is a cross-sectional view illustrating an acceleration sensor 30according to this embodiment.

The acceleration sensor 30 has a circuit board 3. A bore 4 is formed inthe circuit board 3. A sensor chip 5 is mounted on the top surface ofthe circuit board 3 so as to cover the bore 4. An acceleration selectingelement 31 is provided on the circuit board 3.

The acceleration selecting element 31 covers the sensor chip 5 andextends parallel to the circuit board 3 (the x direction in FIG. 5). Theacceleration selecting element 31 has a certain height H from the sensorchip 5. An opening 32 is formed at the right end of the accelerationselecting element 31. The acceleration selecting element 31 serves as acase and is filled with the silicon gel 10. The surface of the silicongel 10 is exposed to the x-direction. The distance L from the sensorchip 5 to the exposed surface of the silicon gel 10 is longer than theheight H.

Experiments have revealed that the detection sensitivity of theconventional acceleration sensor 70 shown in FIG. 16 for detecting anacceleration applied to the acceleration sensor 70 (z direction) isproportionate to the product of the specific gravity d of the silicongel 75 filling the sensor mount 72 and the height of the filling silicongel 75 H₁.

FIG. 7 is a graph showing the relation between the ratio of the length Lof the acceleration sensor 30 to its height H (length/height ratio) andthe ratio of the measured sensitivity of the acceleration component inthe x direction to the measured sensitivity of the accelerationcomponent in the z direction (x/z sensitivity ratio). This graph showsthat the x/z sensitivity ratio is in direct proportion to thelength/height ratio and also that the x/z sensitivity ratio is increasedby increasing the length/height ratio. Extending the length L relativeto the height H enhances the detection sensitivity in the x directionrelative to the detection sensitivity in the z direction. Consequently,the acceleration sensor 30 selectively detects the x-directionacceleration component. The length L is long relative to the height H inthis embodiment.

When an object to which the above acceleration sensor is attachedaccelerates, the acceleration selecting element 31 selects theacceleration component parallel to the circuit board 3, that is, the xdirection acceleration component. The x direction acceleration componentis converted into a pressure. The pressure is transmitted to the sensorchip 5 via the silicon gel 10. The sensor chip 5 receives the pressurefrom the silicon gel 10 and transmits a detection signal correspondingto the pressure.

As described above, the acceleration selecting element 31 extending inthe x direction is provided. The acceleration selecting element 31 isformed in such a manner that the detective sensitivity of the sensorchip 5 in the x direction, which is parallel to the circuit board 3, isenhanced. As in the above embodiments, the device of this embodimenteliminates the necessity for altering the direction of the sensor chip 5to match the direction of an acceleration to be detected.

In contrast to the first and second embodiments, the x directionacceleration component selected by the acceleration selecting element 31is directly transmitted to the sensor chip 5. The pressure transmittingelement 14 is therefore unnecessary in this embodiment. This enablesdetecting an acceleration component in an arbitrary direction with asimpler structure.

(Fourth Embodiment)

A three dimensional acceleration sensor according to a fourth embodimentof the present invention will now be described with reference to FIGS. 8to 10.

To avoid a redundant description, like or same reference numerals of thethird embodiment or FIG. 16 are given to corresponding parts of thisembodiment.

FIG. 8 is a plan view illustrating a three dimensional accelerationsensor 40 according to this embodiment. FIG. 9 is a cross-sectional viewtaken along line 9--9 of FIG. 8. FIG. 10 is a cross-sectional view takenalong line 10--10 of FIG. 8.

The three dimensional accelerator 40 has a circuit board 3. As in thesecond embodiment, bores 4x to 4z are formed in the circuit board 3.Sensor chips 5x to 5z are provided on the top surface of the circuitboard 3 so as to cover the bores 4x to 4z, respectively.

The three dimensional acceleration sensor 40 has a case 41. A firstacceleration selecting element 31x is formed on the case 41. Theacceleration selecting element 31x is formed to cover the sensor chip5x. The acceleration selecting element 31x extends parallel to thecircuit board 3 (the x direction in FIGS. 8 and 10).

A second acceleration selecting element 31y is formed on the case 41.The acceleration selecting element 31y is formed so as to cover thesensor chip 5y. The acceleration selecting element 31y extends parallelto the circuit board 3 (the y direction in FIGS. 8 and 9) and normal tothe acceleration selecting element 31x.

The acceleration selecting elements 31x and 31y are connected. Anopening 42 is formed diagonally at the connecting part. Silicon gel 10is filled into the acceleration selecting elements 31x and 31y throughthe opening 42.

As in the conventional sensor shown in FIG. 16, the case 41 has acylindrical sensor mount 72 surrounding a sensor chip 5z. The sensormount 72 is filled with silicon gel 10. The silicon gel 10 transmits tothe sensor chip 5z a pressure generated when the gel 10 is fluctuated bya z direction acceleration component of the acceleration of the objectand the three dimensional sensor 40 to which it is attached. In otherwords, the sensor mount 72 serves as a third acceleration selectingelement for selecting the z direction acceleration component.

The acceleration selecting elements 31x and 31y, as described in thethird embodiment, select the acceleration components in the x directionand the y direction, respectively. In other words, the accelerationselecting element 31x selects the x direction acceleration component,while the acceleration selecting element 31y selects the y directioncomponent. And, as in the conventional acceleration sensor, the sensormount 72 selects the z direction acceleration component.

The pressures corresponding to the selected components are transmittedto the sensor chips 5x to 5z, respectively. The sensor chips 5x to 5zdetect the pressures, respectively, and issue detection signalscorresponding to each pressure. The signal processor 7 amplifies andprocesses the detection signals and sends them to the outside of thesensor. Acceleration components of the x, y and z directions aretherefore detected separately and detection signals corresponding toeach direction are produced.

The case 41 has a housing 43 surrounding the external terminals 8electrically connected to the circuit board 3. The housing 43 is filledwith sealing material 44, which prevents the external terminals 8 fromflaking off the circuit board 3.

As described above, as in the second embodiment, a simple structureallows a multi-directional acceleration to be separated and detected inthis embodiment. Furthermore, the sensor chips 5x to 5z do not have tobe mounted to point x, y and z directions to match the directions of thecorresponding acceleration components. This reduces the manufacturingcost.

(Fifth Embodiment)

A three dimensional accelerometer sensor according to a fifth embodimentof the present invention will now be described with reference to FIGS.11 to 12.

To avoid a redundant description, like or same reference numerals of thefourth embodiment are given to corresponding parts of this embodiment.

FIG. 11 is a plan view illustrating a three dimensional accelerationsensor 50 according to this embodiment. FIG. 12 is a cross-sectionalview taken along line 12--12 of FIG. 11.

This embodiment is different from the fourth embodiment only by theconfiguration of the acceleration selecting elements 31x and 31y. Theacceleration selecting elements 31x and 31y are connected vertically toeach other as in the fourth embodiment. The connection portion opens inthe z-direction to form an opening 51. The sensor mount 72 opens in thex-direction. The opening 51 therefore opens in the same direction as thesensor mount 72. In the fourth embodiment, when filling the accelerationselecting elements 31x and 31y with the silicon gel, the threedimensional sensor 40 must be held vertically so that the opening 41 ispointed upward. This means that the orientation of the three dimensionalsensor 40 should be different when filling the acceleration selectingelement 31x and 31y from the orientation for filling the sensor mount 72with the silicon gel 10. However, in the three dimensional accelerationsensor 50 according this embodiment, the opening 51 opens in the samedirection as the sensor mount 72. This allows the three dimensionalacceleration sensor 50 to be filled with the silicon gel 10 withoutchanging its orientation. Manufacturing of the acceleration 50 is thusfacilitated.

(Sixth Embodiment)

A three dimensional accelerometer sensor according to a sixth embodimentof the present invention will now be described with reference to FIGS.13 to 15.

To avoid a redundant description, like or same reference numerals of thefourth and fifth embodiments are given to corresponding components ofthis embodiment.

FIG. 13 is a plan view illustrating a three dimensional accelerationsensor 60 according to this embodiment. FIG. 14 is a cross-sectionalview taken along line 14--14 of FIG. 13. FIG. 15 is a cross-sectionalview taken along line 15--15 of FIG. 13. Unlike in the fourth and fifthembodiments, the acceleration selecting elements 31x and 31y of thethree dimensional acceleration sensor 60 are formed separately. Theacceleration selecting elements 31x and 31y have openings 51x and 51y,respectively. The openings 51x and 51y open in the z direction,facilitating the filling of the silicon gel 10 in the accelerationselecting elements 31x and 31y. This facilitates, as in the fifthembodiment, filling of the silicon gel 10 in the acceleration selectingelements 31x and 31y and the sensor mount 72 when manufacturing thethree dimensional acceleration sensor 60. The manufacturing of theacceleration sensor 60 is thus simplified.

Further, the detection sensitivities in the x and y directions areeasily changed by changing the lengths of the acceleration selectingelements 31x and 31y or changing the mass of the silicon gel 10 fillingthe acceleration selecting elements 31x and 31y.

Although only a few embodiments of the present invention have beendescribed so far, it should be apparent to those skilled in the art thatthe present invention may be embodied in many other specific formswithout departing from the spirit or scope of the invention.Particularly, the invention may be embodied in the following forms:

In the above embodiments, the mass of the silicon gel 10 may be changedby diffusing particles having appropriate mass in the gel 10. Theparticles may be of metal, ceramic and synthetic resin. Accordingly, thedetection sensitivity of the acceleration sensors 1, 20, 30, 40, 50 and60 may be adjusted.

In the first and second embodiments, the weights 15, 15x to 15z may beomitted. On the other hand, a weight may be provided in the third tosixth embodiments.

In the first and second embodiments, the sensor chips 5 and 5x to 5z maybe provided on the top surface of the circuit board 3. In the third tosixth embodiments, the cell chips 5 and 5x to 5z may be provided on thebottom surface of the circuit board 3.

In the above embodiments, the acceleration selecting elements 13, 13x to13z, 31x and 31y may extend in arbitrary directions. The structureenables detecting the acceleration component in an arbitrary direction.

In the above embodiments, the sensor chip may have any shape as long asit is of a diaphragm type. For example, sensor chips having an E shapedcross section may be used. Further, other than a diaphragm typesemiconductor, pressure sensors having a C shaped cross section,diaphragm type sensor chips utilizing thin film gauges and diaphragmtype capacitive pressure sensitive sensor chips may be utilized.

In the above embodiments, the acceleration transmitting medium mayinclude any gel material including high molecular gel chieflyconstituted by a material selected from the group consisting of, otherthan silicon gel, polyvinyl alcohol, polyeurethane, polyether andpolyester. The medium may also be constituted by acrylic acidderivative.

In the above embodiments, the weights 15 and 15x to 15z may beconstituted by metal such as copper, brass or iron, gel mixed with metalparticles or rubber.

Therefore, the present examples and embodiments are to be considered asillustrative and not restrictive and the invention is not to be limitedto the details given herein, but may be modified within the scope of theappended claims.

We claim:
 1. An acceleration sensor comprising;a case; a generallyplanar circuit board located in said case; a first pressure sensitivepart mounted on said circuit board; a first acceleration selectingelement having an interior chamber formed to extend parallel to theplane of said circuit board, said first acceleration selecting elementhaving a opening communicating its interior chamber to the outside; amedium substantially filling said interior chamber and contacting saidpressure sensitive part, said medium consisting essentially of a gellike composition; and wherein said acceleration sensor detects acomponent of acceleration in a direction parallel to the plane of thecircuit board of an acceleration of an object to which said accelerationsensor is attached.
 2. The acceleration sensor of claim 1 furthercomprising a pressure transmitting element, said pressure transmittingelement having an interior chamber extending in a directionperpendicular to the plane of said circuit board, said interior chamberof said pressure transmitting element being filled with said medium. 3.The acceleration sensor according to claim 1 further comprising:a secondacceleration selecting element formed in said case, said secondacceleration selecting element having an interior chamber extendingparallel to the plane of said circuit board and perpendicular to theextending direction of the interior chamber of said first accelerationselecting element, said second acceleration selecting element having anopening for communicating its interior chamber to the outside; a secondpressure sensitive part mounted on said circuit board; wherein saidmedium substantially fills the interior chamber of said secondacceleration selecting element and contacts the second pressuresensitive part; a third acceleration selecting element formed in saidcase, said third acceleration selecting element having an interiorchamber extending in a direction perpendicular to the plane of saidcircuit board, said third acceleration selecting element having anopening for communicating its interior chamber to the outside; a thirdpressure sensitive part mounted on said-circuit board; and wherein saidmedium substantially fills said third acceleration selecting element andcontacts said third pressure sensitive part.
 4. The acceleration sensoraccording to claim 3 further comprising;a first pressure transmittingelement having an interior chamber extending in a directionperpendicular to the plane of said circuit board for connecting saidfirst pressure sensitive part and the interior chamber of said firstacceleration selecting element, wherein said medium fills said firstpressure transmitting element; a second pressure transmitting elementhaving an interior chamber extending in a direction perpendicular to theplane of said circuit board for connecting said second pressuresensitive part and the interior chamber of said second accelerationselecting element, wherein said medium fills said second pressuretransmitting element.
 5. The acceleration sensor according to claim 4wherein the interior chambers of each of said first, second and thirdacceleration selecting elements has a cylindrical shape, and further,each of the interior chambers of said first and second pressuretransmitting elements has a cylindrical shape.
 6. The accelerationsensor according to claim 3 wherein at least one of said first, secondand third acceleration selecting elements incorporates a weight in themedium therein.
 7. An acceleration sensor comprising;a generally planarcircuit board; a first pressure sensitive part mounted on said circuitboard; a first acceleration selecting element having an interior chamberextending in a direction parallel to the plane of said circuit board,said first acceleration selecting element, together with said circuitboard, forming a case accommodating said first pressure sensitive part,said first acceleration selecting element having an opening forcommunicating its interior chamber to the outside; a mediumsubstantially filling the interior chamber of said accelerationselecting element, said medium consisting essentially of a gelcomposition; and wherein said acceleration sensor detects a component ofacceleration in a direction parallel to the plane of the circuit boardof an acceleration of an object to which said acceleration sensor isattached.
 8. The acceleration sensor according to claim 7 furthercomprising;a second pressure sensitive part mounted on said. circuitboard; a second acceleration selecting element having an interiorchamber extending parallel to said circuit board and perpendicular tothe extending direction of the interior chamber of said firstacceleration selecting element, said second acceleration selectingelement, together with said circuit board, forming a case foraccommodating said second pressure sensitive part, said secondacceleration selecting element having an opening for communicating itsinterior chamber to the outside, wherein said medium substantially fillssaid second acceleration selecting element; a third pressure sensitivepart mounted on said circuit board; a third acceleration selectingelement having an interior chamber extending perpendicular to saidcircuit board, said third acceleration selecting element, together withsaid circuit board, forming a case for accommodating said third pressuresensitive part, said third acceleration selecting element having anopening for communicating its interior chamber to the outside, whereinsaid medium substantially fills the interior chamber of said thirdacceleration selecting element.
 9. The acceleration sensor according toclaim 8 wherein each of said openings of said first and secondacceleration selecting elements faces a direction perpendicular to theplane of said circuit board.
 10. The acceleration sensor according toclaim 8 wherein the interior chambers of said first and secondacceleration selecting elements are connected to each other at aconnecting part, and a common opening is formed at the connecting part.11. The acceleration sensor according to claim 10 wherein said commonopening of said first and second acceleration selecting elements facesin a direction perpendicular to the plane of said circuit board.
 12. Theacceleration sensor according to claim 7 wherein:said first accelerationselecting element has an inner wall located opposite to said firstpressure sensitive part, and said inner wall is located at a height Hfrom said first pressure sensitive part as measured in a directionperpendicular to the plane of the circuit board; wherein said mediumforms an interface with the atmosphere near said opening in said firstacceleration selecting element, and said interface is spaced from saidfirst pressure sensitive part by a length L as measured in a directionparallel to the plane of the circuit board; and wherein said pressuresensitive part is positioned on said circuit board such that the lengthL is longer than the height H.
 13. An acceleration sensor comprising;agenerally planar circuit board; a first pressure sensitive part mountedon said circuit board, said first pressure sensitive part having agenerally planar thin wall portion that is sensitive to pressurechanges, and wherein the plane of the thin wall portion is arranged tobe parallel to the plane of the circuit board; a first accelerationselecting element having an interior chamber formed to extend parallelto the plane of said circuit board, said first acceleration selectingelement having an opening communicating its interior chamber to theoutside; a medium substantially filling said interior chamber andcontacting said thin wall portion, said medium consisting essentially ofa gel like composition; and wherein said acceleration sensor detects acomponent of acceleration in a direction parallel to the plane of thecircuit board of an acceleration of an object to which said accelerationsensor is attached.